A wide variety of implanted medical devices (IMDs) for delivering a therapy or monitoring a physiologic condition which can employ one or more elongated electrical leads and/or sensors are available. Such IMDs can monitor or deliver therapy to the heart, muscle, nerve, brain, and stomach or other organs. Examples of such IMDs include implantable cardioverter defibrillator devices, which have a pulse generator and one or more electrical leads with one or more electrodes that conduct signals to and receive signals from the patient's heart. These electrical lead(s) and their electrode(s) are placed in or proximate to the organ such that an electrical signal between the electrodes is capable of stimulating the organ. The electrodes may be configured either to deliver a stimulus to the organ, or to detect or sense an intrinsic electrical event associated with the organ.
The leads associated with IMDs typically include a lead body extending between a proximal lead end and a distal lead end that incorporates the one or more exposed electrode or sensor elements located at or near the distal lead end. One or more elongated electrical conductors extend through the lead body from a connector assembly provided at a proximal lead end for connection with associated IMD to the electrode or sensor element located at the distal lead end or along a section of the lead body. Each electrical conductor is typically electrically isolated from other electrical conductors and is encased within an outer sheath insulator, which electrically insulates the lead conductors from body tissue and fluids.
Consideration is taken of various stresses that may be applied to the lead body during an implantation, a lead repositioning procedure, or chronic implanted stresses. For example, continuous flexing of the cardiac lead bodies due to the beating of the heart is an important consideration in maintaining the lead's structural integrity. The effects of lead body damage can progress from an intermittent manifestation to a more continuous effect. In extreme cases, insulation of one or more of the electrical conductors can be breached, causing the conductors to contact one another or body fluids resulting in a low impedance or short circuit. In other cases, a lead conductor can fracture and exhibit an intermittent or continuous open circuit resulting in an intermittent or continuous high impedance. Such lead issues resulting in short or open circuits, for example, can be referred to, for simplicity, as “lead-related conditions.”
In the case of cardiac leads, the ability to sense cardiac activity conditions accurately through a lead can be impaired by these lead-related conditions. Complete lead breakage impedes any sensing functions while lead conductor fractures or intermittent contact can demonstrate electrical noise that interferes with accurate sensing. During cardiac pacing or defibrillation, lead-related conditions can reduce the effectiveness of a pacing or defibrillation pulse or therapy to below that which is sufficient to pace or defibrillate the heart.
It is generally desirable to provide mechanisms to sustain therapy delivery in the event of a lead-related condition.